1. Field of the Invention
This invention relates generally to patterned-media magnetic recording disks, wherein each data bit is stored in a magnetically isolated data island on the disk, and more particularly to patterned-media disks for multi-track recording.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, the magnetic recording layer on the disk is patterned into small discrete magnetic data islands separated by nonmagnetic spaces and arranged in concentric data tracks. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic.
One proposed method for fabricating patterned-media disks is by nanoimprinting with a disk or template, sometimes also called a “stamper”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the template. The polymer film receives the reverse image of the template pattern and then becomes a mask for subsequent etching of the disk substrate to form the pillars on the disk. In one type of patterned media, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with nanoimprinting is then pressed on top of these layers. The polymer film receives the reverse image of the mold pattern and then becomes a mask for subsequent milling, etching or ion-bombarding the underlying layers. In another type of patterned media, the magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate and the tops of the pillars to form the patterned-media disk. The template may be a master template for directly imprinting the disks. However, the more likely approach is to fabricate a master template with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master template to fabricate replica templates. The replica templates will thus have a pattern of recesses or holes corresponding to the pattern of pillars on the master template. The replica templates are then used to directly imprint the disks.
In the proposed patterned disks, the data islands are equally spaced along single data tracks with the data tracks being equally spaced in the radial or cross-track direction. The read and write heads read and write the data islands on a single track so the data islands are typically sized to generally match the lateral or cross-track width of the heads. The data islands are spaced to define a bit aspect ratio (BAR). The BAR is the ratio of track spacing (TS) or pitch in the radial or cross-track direction to the island spacing (IS) or pitch in the circumferential or along-the-track direction, which is the same as the ratio of linear island density in bits per inch (BPI) in the along-the-track direction to the track density in tracks per inch (TPI) in the cross-track direction. To minimize the resolution requirement for fabricating the islands, it is preferable that the array of islands have a low BAR (about 1). However, it is difficult to fabricate read/write heads with the proper performance for very narrow tracks with data islands having this low BAR. Also, if the single data tracks are too closely spaced, islands in tracks adjacent to the track being written may also be written by fringing fields from the write head, and the readback signal from a data track being read may receive interference from data recorded in adjacent tracks.
To address these problems a patterned-media disk drive for multi-track recording has been proposed. This allows the heads to be wider, which makes them easier to fabricate, and also allows the drive to read and write two tracks at a time, thereby doubling the data rate and bringing the performance closer to conventional disk drives. For a two-track recording disk drive, the islands in each track are shifted in the along-the-track direction by ½ times the island spacing (IS) from the islands in adjacent tracks. The read and write heads have a lateral or cross-track width to allow writing and reading two tracks at a time without excess interference from tracks beyond the pair being written or read. The heads read or write all the data islands in two tracks by alternately reading or writing the islands from each of the tracks in succession. This type of patterned media multi-track recording disk drive is described in U.S. Pat. No. 6,937,421 and U.S. Pat. No. 7,782,561.
The proposed patterned-media disk for multi-track recording has circular islands arranged in a hexagonal-close-packed (hcp) pattern. This results in a low BAR of 0.87, which is preferred because of ease of lithography and fabrication. However, a low BAR results in a very narrow track pitch or spacing (TS) which makes fabrication of the read and write heads difficult. A low BAR also stresses the track following servo system because of very narrow tracks, and lowers the data rate due to relatively low linear (along-the-track) bit density. An additional problem with multi-track recording, especially with circular islands in an hcp pattern, arises because of head “skew”. Because the read head is fixed at the tip of a rotary actuator, it is not always orthogonal to the tracks but makes an angle with the tracks as it moves in an arcuate path across the disk. This can result in the read head reading islands from adjacent tracks simultaneously, rather than alternately as required for multi-track recording.
What is needed is a patterned-media disk for multi-track recording with a data island pattern that address the problems associated with multi-track recording.